Method for producing a bolometric detector

ABSTRACT

A method for producing a bolometric detector comprising producing a stack, on an interconnect level of a read-out circuit, comprising a sacrificial layer positioned between a carrier layer and an etch stop layer, the sacrificial layer comprising a mineral material; producing a conducting via passing through the stack such that it is in contact with a conducting portion of said interconnect level; depositing a conducting layer onto the carrier layer and the via; etching the conducting layer and the carrier layer, forming a bolometer membrane electrically connected to the via by a remaining portion of the conducting layer that covers an upper part of the via; and elimination of the sacrificial layer by selective chemical etching, and such that the membrane is suspended by the via.

TECHNICAL FIELD AND PRIOR ART

The invention relates to the field of bolometer or microbolometerelectromagnetic radiation detectors intended to detect radiation in the“thermal” or infrared region.

The term “bolometer” is used in a generic manner herein, regardless ofthe dimensions of the bolometer, and thus refers to both a bolometer ora microbolometer of micrometric dimensions.

A bolometer detector, or bolometric detector, generally comprises a setof bolometers positioned at the surface of a carrier substrate to forman array. Each of the bolometers is intended to provide thermalinformation relative to a pixel of an image provided by the detector.

Each bolometer comprises a suspended membrane mechanically andelectrically connected to the substrate by means of long narrow beamscalled support arms and connected to electrically conducting pillarsenabling the membrane to be suspended. The assembly is placed in asealed enclosure, for example a casing under very low pressure, in orderto suppress the thermal conductance of the surrounding gas.

During operation of the detector, the membrane of each bolometer heatsup by absorbing the incident radiation originating from the observedthermal scene, said radiation being transmitted and focused on thebolometer array by an appropriate optical system at the level of thefocal plane array. Said membrane comprises a thermoelectric signaltransduction device having an electric property (for exampleresistivity) that strongly varies when the temperature changes, forexample generating a current variation, i.e. an electric signal, whensaid material is under a constant biasing, the amplitude of which is afunction of the incident radiation flow. The signal obtained correspondsto the image of the temperature of the detector. Such a thermoelectricsignal transduction device corresponds, for example, to a layer oftransducer material, for example a metal oxide (VO_(X), NiO_(X),TiO_(X)) or amorphous silicon (a-Si), or a diode or a transistor.

Conventional methods of manufacturing such detectors, of the “above IC”or “MEMS on top” type, comprise steps directly carried out at thesurface of a substrate generally made of silicon and comprising aplurality of electronic circuits forming the read-out integrated circuitor ROIC, in a so-called monolithic fashion. The term “monolithic” refersto a continuous sequence of operations carried out using the samesubstrate, after the read-out integrated circuit manufacturing process.

The steps for manufacturing bolometers are generally carried out tocollectively manufacture a plurality of detectors, for example from afew tens to a few hundred thereof on the same substrate.

During these steps, the bolometer elements implementing the radiationabsorption, optothermal signal transduction, and thermoelectric signaltransduction functions are positioned at the surface of a sacrificiallayer intended to form a construction base and to be removed at the endof the process by an appropriate method that does not attack the otherelements (the structural and active members) of the structure. Thesacrificial layer generally corresponds to an organic polyimide layer,which is then removed by combustion in an oxygen plasma.

The dielectric or semi-conductive layers that form the elements of thedetection structure are conventionally made of silicon oxide (SiO),silicon nitride (generically written SiN) or amorphous silicon, asdisclosed, for example, in document EP 0 828 145 A1. These materials canbe deposited at a relatively low temperature and are inert relative tothe method of removing the organic sacrificial layer carried out underan oxygen plasma, that is to say they are not etched by the oxygenplasma.

After etching the sacrificial layer, the membranes of the bolometersremain suspended above the substrate without any contact or attachmentother than the support arms thereof.

The bolometers are, for example, made by forming an array with arepeating pitch (distance separating the centres of two neighbouringbolometers positioned in the same row or same column of the array) equalto 17 μm, or even of about 12 μm or 10 μm.

In order to carry out a “far” infrared detection (LWIR), the detectorgenerally comprises a quarter-wave plate or cavity formed between themembrane and a reflector positioned at the surface of the substrate, inorder to provide the detector with a maximum level of absorption forwavelengths in the vicinity of 10 μm. Thus, in order to connect and holdthe membrane at a suitable distance from the reflector and with goodmechanical support, the electrically conducting pillars produced arerather complex and have non-negligible overall dimensions, and areformed through a thick polyimide layer (generally having a thicknessthat lies in the range 2 μm to 2.5 μm) forming the sacrificial layer,the thickness of said sacrificial polyimide layer corresponding to thedesired distance between the membrane and the reflector.

Document EP 2 743 659 A1 proposes to partially integrate the bolometersinto the “back-end” layers (or “BEOL” for “Back End Of Line”) of theread-out integrated circuit of the bolometers. The acronym BEOL refersto the steps of manufacturing all metal interconnections carried out ata relatively low temperature, typically less than about 400° C., andwhich are characteristic of the end of standard microelectronicmanufacturing processes. The purpose of this so-called “MEMS-in-CMOS”approach is to use certain BEOL layouts that are mature on an industriallevel to integrate part of the bolometer elements. In particular,metallised vias obtained using a “damascene” method, are produced toform the electrically conducting pillars of the bolometers, and one ofthe IMDs (“Inter-Metal-Dielectrics”), for example comprising SiO, whichis a standard material in microelectronics, is used as a sacrificiallayer on which the membranes of the bolometers are produced.

The removal of such a sacrificial layer requires the use of vapour-phasehydrofluoric acid (HFv). Therefore, all materials forming the bolometerare chosen to be inert relative to this very chemically aggressivemethod, i.e. from the group of materials not affected by this etching.

By integrating bolometers into the BEOL, the last photolithographiclevels of the ROIC normally carried out to conduct an electrical contactat the surface of the passivation layer located at the apex of the BEOLare also carried out to produce the electrically conducting pillars ofthe bolometers. A plurality of lithographic levels in the group oflevels required to manufacture bolometers are thus avoided, whichresults in significant savings in the manufacturing costs of saidbolometers. Moreover, the electrically conducting pillars, cylindricalin shape, thus produced benefit from the CMOS routing rules, i.e. adiameter equal to about 0.5 μm, which represents very significantcompactness savings compared to the electrically conducting pillarsgenerally produced using “above IC” technology, which generally occupy asurface area of about 3×3 μm². These space savings can represent adecisive advantage in a context of reducing the pitch of bolometricarray detectors.

In the structure disclosed by document EP 2 743 659 A1, the electricalconnection between the electrically conducting pillars and the metalpresent in the membrane of the bolometer in order to ensure absorptionof the electromagnetic radiation can be obtained indirectly via acarrier layer of the membrane comprising amorphous silicon. However, theelectrical resistivity of this non-metallic amorphous silicon layer isvery high compared to metal and introduces, despite its small thickness,a parasitic electrical resistance in series with the thermoelectricsignal transducer element, which is detrimental to the sensitivity ofthe micro-detectors.

In order to overcome this restriction, document EP 2 743 659 A1 furtherdiscloses another embodiment wherein an electrical contact is providedthrough the carrier layer of the membrane made of amorphous silicon bymeans of an opening made through said carrier layer, in order toovercome the aforementioned drawbacks. This contact, and therefore theopening in which this contact is made, must fall within thecross-section of the electrically conducting pillar so that the metallayer present in the membrane of the bolometer can create a reliableelectrical continuity with the metal forming the electrically conductingpillar. This arrangement reveals two key points:

the definition of the contact by photolithography and the alignmentthereof on the electrically conducting pillar is very delicate andsometimes impossible when the diameter of the electrically conductingpillar is reduced to the minimum dimension allowed by thephotolithography means, however which constitutes one of the mainadvantages of this “MEMS in CMOS” construction of the bolometer. Morespecifically, producing a contact inside a section, the diameter ofwhich is equal to 0.5 μm requires the use of advanced lithographic meansnot available in a conventional BEOL line,

the contact is made through the amorphous silicon layer, the thicknessof which is generally equal to 50 nm. The resulting topography must notcreate any discontinuity in the metal layer of the bolometer depositedin the contact. This layer, which must be very thin (typically 10 nm)since it also serves as a radiation absorber in the bolometric membrane,will not be very effective in covering the sides of the contact etchedin the amorphous silicon carrier layer.

Moreover, another drawback of this solution concerns the addition of alithographic and etching level for producing the opening in which thecontact must be produced, which generates additional manufacturingcosts.

DESCRIPTION OF THE INVENTION

Thus there is a need to propose a method for producing a bolometricdetector wherein the one or more bolometers are partially producedwithin the BEOL of the electronic read-out circuit of the detector, andwhich overcomes the aforementioned drawbacks concerning the electricalcontact between the membrane of a bolometer and at least one of theelectrically conducting pillars from which the membrane of saidbolometer is suspended.

For this purpose, one embodiment proposes a method for producing abolometric detector comprising at least the implementation of thefollowing steps of:

producing a stack of layers on an electrical interconnect level of anelectronic read-out circuit of the detector, the stack comprising atleast one sacrificial layer positioned between a carrier layer and afirst etch stop layer, the first etch stop layer being positionedbetween the sacrificial layer and said electrical interconnect level,and the sacrificial layer comprising at least one mineral materialcapable of being selectively etched relative to the carrier layer andthe first etch stop layer;

producing at least one electrically conducting via passing through atleast the stack of layers such that at least one electrically conductingmaterial of the via is in contact with at least one electricallyconducting portion of said electrical interconnect level connected tothe electronic read-out circuit;

depositing at least one electrically conducting layer onto the carrierlayer and the via;

etching the electrically conducting layer and the carrier layer, forminga bolometer membrane electrically connected to the electricallyconducting via by at least one remaining portion of the electricallyconducting layer that covers at least one upper part of the via;

eliminating the sacrificial layer by chemical etching to which the firstetch stop layer and the carrier layer are resistant, and such that themembrane is suspended by means of the via.

In this method, the one or more bolometers are produced on an electricinterconnect level of the electronic read-out circuit and by using amineral sacrificial layer, and are thus partially integrated into theBEOL of said circuit.

Moreover, due to the fact that the electrically conducting layer isdeposited directly on the electrically conducting via acting as a pillarfor suspending the membrane, and that the electrically conducting via isproduced after having formed the carrier layer, the electrical contactbetween the electrically conducting layer of the membrane and theelectrically conducting via is made directly between said two elements,without the presence of any semi-conductive material therebetween.

This method is used to form a direct electrical connection between theelectrically conducting via and the electrically conducting layer of themembrane which benefits from lower overall dimensions due to the factthat a large contact area of the electrically conducting pillar is notrequired to perform the alignment with an opening formed through thecarrier layer as is the case in the prior art. The electricallyconducting via can therefore be produced with smaller dimensions than inthe prior art, for example with a diameter or a side, the dimensions ofwhich are about 0.5 μm.

Moreover, since the electrical contact between the electricallyconducting via and the electrically conducting layer is not formed in anopening passing through the carrier layer, a small thickness of theelectrically conducting layer is not detrimental to the reliability ofthe contact.

Finally, with regard to the method requiring the production of anopening passing through the carrier layer of the membrane in order toproduce the contact between the electrically conducting pillar and theelectrically conducting layer of the membrane, this method furtherallows a level of photolithography and etching to be avoided, due to thenon-production of such an opening in said method.

This method is used to obtain excellent electrical continuity betweenthe read-out circuit and the membrane of the bolometer since theelectrical connection between the read-out circuit and the electricallyconducting material of the membrane is solely formed of electricallyconducting materials.

The above advantages are obtained, regardless of the material of thecarrier layer.

The stack of layers may further comprise a second etch stop layer suchthat the carrier layer is positioned between the second etch stop layerand the sacrificial layer, the method may further comprise, between theproduction of the via and the deposition of the electrically conductinglayer, the implementation of the following steps of:

removing, for example by chemical-mechanical polishing, a layer ofelectrically conducting material formed on the second etch stop layerduring production of the via, then

eliminating the second etch stop layer,

and the electrically conducting layer can be deposited such that theremaining portion of the electrically conducting layer also covers thesides of the upper part of the via uncovered (or bared or revealed) bythe elimination of the second etch stop layer.

Since the upper part of the electrically conducting via “protrudes” fromthe carrier layer, and therefore since the mechanical holding of thecarrier layer by the via takes place beneath this upper part, themechanical holding of the carrier layer, and therefore of the membrane,to the electrically conducting via is improved. Furthermore, since theremaining portion of the electrically conducting layer covers, inaddition to the top surface of the via, the sides of said upper part ofthe via, the contact area between this portion of the electricallyconducting layer and the electrically conducting via increases, and theelectrical contact between the electrically conducting layer and theelectrically conducting via is thus improved.

The carrier layer may comprise at least one dielectric material or amaterial wherefore at least one electric parameter varies according tothe temperature thereof. Therefore, the carrier layer performs anelectrical insulation or thermoelectric signal transduction functionwithin the membrane of the bolometer.

The method may further comprise, before the elimination of thesacrificial layer, the production of at least one element for absorbingthe infrared radiation intended to be detected by the detector, on themembrane.

In such a case, the absorbing element may comprise at least one MIM (or“Metal-Insulator-Metal”) structure. Other types of absorbing elementscan be produced on the membrane.

The expression “MIM structure” refers to a stack comprising at least onedielectric element positioned between an upper metal element and a lowermetal element, and capable of performing the selective absorption ofcertain wavelengths according to the dimensions and materials of the MIMstructure.

The electrically conducting layer may be etched such that a plurality ofremaining portions of said electrically conducting layer form bolometerelectrodes and resistive portions capable of absorbing infraredradiation intended to be detected by the detector. These resistiveportions can take on any shape.

The method may further comprise, between the etching of the electricallyconducting layer and the etching of the carrier layer (the carrier layerbeing etched after the electrically conducting layer), the deposition ofa first thermoelectric signal transduction layer on the remainingportions of the electrically conducting layer and on the carrier layer.Such a first thermoelectric signal transduction layer may be depositedon the electrically conducting layer and on the carrier layer. Inaddition to the electrical connection between the conducting layer andthe thermoelectric signal transduction layer, this arrangement alsocompensates for a bimetal effect occurring between the portions of theelectrically conducting layer and the carrier layer.

The first thermoelectric signal transduction layer may comprise at leastone material that is resistant to the chemical etching processimplemented to eliminate the sacrificial layer. In such a case, thisfirst thermoelectric signal transduction layer can also act to protectthe electrically conducting layer relative to the etching solution usedto etch the sacrificial layer.

The method may further comprise, after the deposition of the firstthermoelectric signal transduction layer, the deposition of a secondthermoelectric signal transduction layer on the first thermoelectricsignal transduction layer, the thickness of which is greater than thatof the first thermoelectric signal transduction layer. In such a case,the signal-to-noise ratio of the conversion of the heat energy absorbedinto electrical energy taking place in the detector is improved. Thesecond thermoelectric signal transduction layer may comprise at leastone material that is resistant to the chemical etching processimplemented to eliminate the sacrificial layer.

This second thermoelectric signal transduction layer may then be etchedin order to form one or more portions of thermometric material on themembrane, for example a central portion of thermometric material.

The method can be such that:

the carrier layer comprises amorphous silicon, and/or

if a first thermoelectric signal transduction layer is deposited, thefirst thermoelectric signal transduction layer comprises amorphoussilicon, and/or

if a second thermoelectric signal transduction layer is deposited, thesecond thermoelectric signal transduction layer comprises amorphoussilicon.

BRIEF DESCRIPTION OF THE FIGURES

This invention will be better understood after reading the followingdescription of embodiments, given for purposes of illustration only andnot intended to limit the scope of the invention, and with reference tothe accompanying figures, wherein:

FIGS. 1A to 1N show the steps of a method for producing a bolometricdetector according to a first embodiment; and

FIG. 2 shows a bolometric detector obtained by implementing a productionmethod according to a second embodiment.

Identical, similar or equivalent parts of the different figuresdescribed herein below carry the same numerical references in order toease the passage from one figure to another.

The different parts shown in the figures are not necessarily displayedaccording to a uniform scale in order to make the figures easier toread.

The different production possibilities (alternatives and embodiments)must be understood as not being exclusive with regard to each other andcan be combined together.

DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENTS

A method for producing an infrared bolometric detector 100 according toa first embodiment is described herein below with reference to FIG. 1Ato 1N.

The device 100 produced in this first embodiment comprises a pluralityof thermal detectors, of the microbolometer type, produced from asemi-conductive substrate 10, for example silicon, on and/or in which isintegrated an electronic read-out circuit 12 made using CMOS technology.The read-out circuit 12 reads the electrical characteristic variationinduced by the heating of each of the bolometers, and also the biasingof the bolometers.

The electronic read-out circuit 12 comprises semi-conductor layers 13(“Front End” part) in which transistors, diodes, capacitors and otherMOS-type electronic devices are produced, allowing the electronicfunctions of the read-out circuit 12 to be performed. One or moreelectrical interconnect levels 14 (“Back End” part) in particularconnecting functional units of the read-out circuit 12 together andintended to form input/output connections of the read-out circuit 12,are produced on the semi-conductor layers 13 of the read-out circuit 12.

The last electrical interconnect level intended to form the electricalcontacts of the read-out circuit 12 has not yet been produced at thestage shown in FIG. 1A. In FIG. 1A, the reference numeral 14 cantherefore designate a single electrical interconnect level,corresponding to both the first electrical interconnect level and to thepenultimate electrical interconnect level, or a plurality of stackedelectrical interconnect levels from the first electrical interconnectlevel (that in contact with the semi-conductor layers 13) to thepenultimate electrical interconnect level. The last electricalinterconnect level will be produced such that it integrates thebolometers intended to be produced such that it is suspended above theread-out circuit 12 and provides the electrical connections between theinputs of the read-out circuit 12 accessible from the electricalinterconnect levels 14 and the output electrical connections of thebolometers. This last electrical interconnect level is also producedsuch that it forms electrical connection pads of the read-out circuit 12accessible externally, i.e. capable of being electrically contacted fromoutside the detector 100.

Alternatively, a plurality (for example 2 or 3) of last electricalinterconnect levels can be used to form the mechanical support means ofthe bolometers and the electrical connection means between the inputs ofthe read-out circuit 12 and the output electrical connections of thebolometers.

FIG. 1B shows the penultimate electrical interconnect level 20 produced,forming a part of the electrical interconnect levels 14 and on which thebolometers of the detector 100 are intended to be produced. Thiselectrical interconnect level 20 in particular comprises a metal line21, the portions 16 of which (a single portion 16 is shown in FIG. 1B)are electrically and locally connected, by means of electricallyconducting vias 22, or vertical connections, and possibly by means ofthe one or more lower electrical interconnect levels, to the MOSelectronic devices of the read-out circuit 12. At least one part ofthese portions 16 are intended to be electrically connected to thebolometers of the detector 100. Outside of the vias 22 which provide theelectrical connections with the lower electrical interconnect level, theelectrical interconnect level 20 is electrically insulated from themetal line of the lower electrical interconnect level by a dielectriclayer 27, called an inter-metal dielectric (or IMD), which is a mineralmaterial and for example, composed of undoped silicon oxide (also calledUSG or “Undoped Silica Glass”), or of an oxide with a lower dielectricpermittivity, such as SiOF, SiOC or SiOCH, etc.

The electrical interconnect level 20 can further comprise other portions(not shown in FIG. 1B) of the metal line 21, connected or not connectedto the lower electrical interconnect level, intended to not be connectedto the bolometers but to be used, for example, to produce, at leastpartially, connection pads used for the wiring and testing of theread-out circuit 12 and detector 100 in general.

Different technical options are available for producing the vias 22 andthe metal line 21 of the penultimate electrical interconnect level 20(and also the metal lines and vias of the other electrical interconnectlevels 14). A first possibility consists of producing the metal line 21using aluminium advantageously inserted between two layers of titaniumor titanium nitride, and of producing the vias 22 using tungsten. Asecond possibility involves producing the metal line 21 and the vias 22using copper according to a damascene method comprising filling trenchesformed in the dielectric layer 27, or inter-metal dielectric, withcopper. This can be a simple damascene method wherein two successivedamascene steps are performed to produce the vias 22 then the metal line21, or a dual damascene method as shown in FIG. 1B, wherein the steps ofproducing the metal line 21 and the vias 22 are integrated into a“continuous” manufacturing process. The core of said elements 21 and 22is formed from copper portions 23 produced by the electrolysis of acopper salt solution (ECD). The bottom surfaces and the side surfaces ofsaid copper portions 23 are further coated in a set of conductivelayers, for example made of tantalum nitride 24, tantalum 25 and copper26, advantageously produced by ionising cathodic sputtering (also callediPVD) in order to improve the coating of said deposits on the verticalsides. The tantalum layer 25 forms a barrier layer preventing thediffusion of the copper 23 and 26 into the inter-metal dielectric 27 andto the semi-conductor layers 13 of the read-out circuit 12, where, ifnot prevented, the copper would create electrical defects, in particularin the transistors and diodes. The tantalum nitride layer 24 isgenerally provided to improve adhesion of the tantalum layer 25 to thesurface of the inter-metal dielectric 27. The copper layer 26 acts bothas a cathode and as a seed layer for the electrolytic growth of thecopper portions 23. The flush-fitting of the interconnect level 20 withthe surface of the inter-metal dielectric 27 is obtained bychemical-mechanical polishing (CMP) controlled by a stop layer 28, forexample comprising SiO₂.

An insulating dielectric layer 29 covers the stop layer 28 and the topsurfaces of the different portions of the metal line 21. This layer 29corresponds, in this case, to a bilayer formed by stacking a first layerpositioned on the stop layer 28 and a second layer covering said firstlayer. The first layer of said stack is intended to form a diffusionbarrier with regard to the copper of the metal line 21, and comprises,for example, silicon nitride. The second layer of the stack forms anetch stop layer capable of withstanding HFv etching implemented at alater time during the liberation of the membranes of the bolometers,which will thus protect the electrical interconnect level 20 during theliberation of the bolometers. This second layer of the stack comprises,for example, Al₂O₃ or AlN.

In the following figures, the metal line 21 and the vias 22 are shown ina less detailed manner than in FIG. 1B for easier reading.

A sacrificial layer 30, comprising at least one mineral material such asSiO₂, is deposited on the layer 29 (FIG. 1C). The thickness (dimensionalong the Z axis) of the sacrificial layer 30 lies, for example, in therange 1 μm to 5 μm approximately. The thickness of the sacrificial layer30 is in particular chosen as a function of the desired absorptionproperties relative to infrared radiation intended to be received andabsorbed by the bolometers of the detector 100. This thickness is, forexample, chosen such that the space freed by the subsequent etching ofthe sacrificial layer 30 forms, beneath the membrane of the bolometers,a quarter-wave cavity (in particular when the detector 100 is intendedto detect infrared radiation, the wavelength of which falls within bandIII for example (between 8 and 12 μm)). The thickness of the sacrificiallayer 30 can be adjusted, after the deposition thereof, by theimplementation of CMP-type planarization.

As shown in FIG. 1D, a carrier layer 32 is then deposited on thesacrificial layer 30. The carrier layer 32 can comprise at least onetemperature-sensitive material, for example the resistivity whereofvaries substantially with the temperature. In such a case, the carrierlayer 32 comprises, for example, amorphous silicon deposited by CVD.Alternatively, the carrier layer 32 can comprise at least one materialforming an electrical insulator or with significant resistivity.Therefore, in addition to the amorphous silicon, the carrier layer 32can comprise SiC, Al₂O₃, or AlN, etc. In all cases, the one or morematerials of the carrier layer 32 are inert relative to the HFv etchingimplemented at a later time during the etching of the sacrificial layer30, i.e. they are capable of withstanding (not being etched) the one ormore chemical etching solutions used to etch the sacrificial layer 30.Therefore, the material of the sacrificial layer 30 is capable of beingselectively etched relative to the carrier layer 32 and the first etchstop layer 29.

Moreover, the carrier layer 32 can correspond to a stack of a pluralityof different materials, for example a bilayer, such that the lower layerof said stack (that in contact with the sacrificial layer 30) protectsthe one or more other upper layers of said stack during the etching ofthe sacrificial layer 30.

The thickness of the carrier layer 32 lies, for example, in the range 10nm to 100 nm approximately.

A second etch stop layer 34 is then deposited on the carrier layer 32(FIG. 1E). This layer 34 is intended to stop an ablation orplanarization process implemented at a later time to remove excessconductive material formed during the production of electricallyconducting vias connecting the bolometers to the metal line 21, in orderto prevent deterioration of the layer 32 during this removal ofconductive material. The layer 34 comprises at least one material thatis resistant to mechanical and/or chemical and/or ionic abrasion thatwill be implemented, preferably corresponding to at least one materialalready used during the production of the elements found in thesemi-conductor layers 13, for example SiN and/or SiO. The layer 34 has,for example, a thickness of between about 20 nm and 100 nm.

Electrically conducting vias 38 are then produced through the stack oflayers previously formed on the electrical interconnect level 20. FIG.1F to 1H describe the production of a single electrically conducting via38. However, a plurality of electrically conducting vias are producedthrough said stack. The number of said vias 38 depends on the number ofbolometers that the detector 100 is intended to contain. Saidelectrically conducting vias 38 are intended to provide the electricalconnections between the bolometers of the detector 100 and the read-outcircuit 12, as well as to mechanically hold the membranes of thebolometers in suspension.

As shown in FIG. 1F, an opening 36 is etched through the stack formedfrom the layers 34, 32, 30 and 29 previously produced. Said opening 36forms an access to the metal line 21 and is intended for the productionof one of the electrically conducting vias 38. The shape and dimensionsof the section of the opening 36, in a plane of the layers 34, 32, 30and 29 at the interfaces between said layers through which passes saidopening 36, define the shape and the dimensions of the via 38 that willbe produced in said opening 36, whereby said shape can be, for example,circular or polygonal, the diameter or a dimension of one side of saidopening capable of being equal to about 0.5 μm.

The opening 36 is then filled with one or more electrically conductingmaterials, thus forming the electrically conducting via 38. A thinbarrier layer, comprising for example TiN, is for example depositedagainst the walls of the opening 36 (at the bottom and against the sidewalls), then the remaining empty volume inside the opening 36 is filledwith another electrically conducting material, for example tungstendeposited by CVD. The barrier layer present at the bottom of the opening36, between the metal line 21 and the other electrically conductingmaterial positioned in the opening 36, prevents a chemical reaction fromtaking place between the electrically conducting material (for examplecopper) of the metal line 21 and said other electrically conductingmaterial (for example tungsten) deposited in the opening 36. Saidbarrier layer present against the side walls of the opening 36 furtherimproves the adherence of said other electrically conducting materialdeposited in the opening 36.

Alternatively, the metallisation of the electrically conducting via 38can be obtained using copper. The aforementioned different embodimentsof the metal line 21 and of the vias 22 can apply for the production ofthe electrically conducting via 38.

After the deposition of the one or more electrically conductingmaterials in the opening 36, a layer 39 of said one or more electricallyconducting materials deposited to form the electrically conducting via38 is present on the layer 34 (FIG. 1G). A removal step, for example byCMP, is implemented in order to remove said layer 39, whereby the layer34 acts as a stop layer stopping said removal (FIG. 1H).

The layer 34 is then removed (FIG. 1I), for example by chemical etchingin an aqueous environment, providing a very high etching selectivityrelative to the carrier layer 32 underlying the layer 34 and relative tothe one or more electrically conducting materials of the electricallyconducting via 38. Such an etching calls on, for example, the use of aBOE-type (“Buffered Oxide Etch”) etching solution with a diluted HFbase. The etching agents are chosen such that the carrier layer 32 isnot etched by said etching agents, which can be of the same kind asthose used at a later time to etch the sacrificial layer 30. Saidetching of the layer 34 reveals an upper part 40 of the electricallyconducting via 38 (symbolically delimited from the rest of the via 38 bya dotted line), the sides of which are not covered, at this stage of themethod, by any material. The thickness of said upper part 40 is equal tothat of the layer 34.

An electrically conducting layer 42 is then deposited on the carrierlayer 32 and further covers the upper part 40 of the electricallyconducting via 38, and in particular the top surface and the sidesurfaces of said upper part 40 of the electrically conducting via 38(FIG. 1J). The layer 42 comprises, for example, TiN. This covering ofthe upper part 40 of the electrically conducting via 38 by the layer 42increases the contact area between the conductive material of the layer42 and the one or more conductive materials of the via 38, and thusimproves electrical conduction between the pillar formed from the via 38and the layer 42. When the membrane of the bolometer produced isliberated by etching the sacrificial layer 30, the fact that the carrierlayer 32 surrounds the via 38 at a level located beneath said upper part40 of the via 38 will increase the mechanical robustness of theassembly.

The bolometer is then completed by producing the different optothermaland thermoelectric signal transduction elements on the carrier layer 32,by etching the carrier layer 32 (and any possible other layers depositedon the carrier layer 32) to form the membrane of the bolometer, and byetching the sacrificial layer 30 in order to liberate said membrane.

In the first embodiment described herein, the layer 42 is etched suchthat the remaining portions of this layer form electrodes 44 of thebolometer in direct contact with the one or more conductive materials ofthe vias 38 (a single electrode 44 is shown in FIG. 1K). Other remainingportions of the layer 42 form resistive portions 46 capable of absorbingthe infrared radiation intended to be detected by the detector 100. Theportions of the carrier layer 32 made of amorphous silicon that are notcovered by the electrodes 44 and the resistive portions 46 will formthermoelectric signal transduction regions of the bolometer. Theelectrodes 44 and the resistive portions 46 can have different shapes.By way of example, the resistive portions 46 can correspond to two partsof the layer 42 separated from one another by a slot etched through thelayer 42 such that current cannot pass between said portions 46. Othershapes and/or a different number of resistive portions 46 can beconsidered.

As shown in FIG. 1L, a thermoelectric signal transduction layer 48,called a first thermoelectric signal transduction layer, is deposited onthe remaining portions 44, 46 of the layer 42 and on the parts of thecarrier layer 32 not covered by said portions 44, 46. Said layer 48, forexample comprising amorphous silicon, is intended to ensure, with thecarrier layer 32 (since this is also made from amorphous silicon in thisfirst embodiment), the conversion of the infrared radiation received bythe bolometer into electrical energy. Furthermore, the layers 32 and 48compensate for a bimetal effect occurring between the portions 46 of theelectrically conducting layer and the carrier layer 32.

A second thermoelectric signal transduction layer 50 is then depositedon the layer 48 (FIG. 1M). Advantageously, said layer 50 comprisesamorphous silicon and has a thickness that is greater than that of thelayer 48 and that of the carrier layer 32.

Said second layer 50 is etched such that at least one remaining part 52of said layer 50 is capable of forming at least one thermoelectricsignal transduction element of the bolometer. In the first embodimentdescribed herein, the remaining part 52 forms a central thermometricelement of the membrane 54. Advantageously, the parts of the secondlayer 50 located on the areas intended to form the support arms of themembrane 54 are removed during said etching, which provides the membrane54 with a high level of thermal insulation. The process of etching thesecond layer 50 can advantageously involve the use of an etch stoplayer, which in particular is used to control the thickness in thesupport arms.

The layers 48 and 32 are then etched according to the desired pattern inorder to form the membrane 54 of the bolometer and the support arms ofthe membrane 54. Finally, the sacrificial layer 30 is chemically andselectively etched relative to the materials of the layers 32, 48, 50and 29, thus liberating the membrane 54 that is suspended thanks to thevias 38 (FIG. 1N).

The infrared thermal detector 100 obtained is formed from a centralelement comprising, in addition to the absorbing elements 46 producedfor example in the form of metal strips, thermometric elementscorresponding to the superimposed portions of temperature-sensitivematerials derived from the three layers 32, 48, 52, the overallthickness of which is substantially greater than the sum of thethicknesses of the layer 32 and 48. In such a case, this results inreduced electrical noise from the thermometric elements according to alaw that is inversely proportional to the thickness of the thermometricelements. Said central element is electrically and mechanicallyconnected to the electrically conducting vias 38 by means of arms formedby superimposing the materials of the layers 32, 42 and 48, thethickness of said arms being substantially less than that of the centralelement. This results in reduced thermal conductance of said arms and,thus, in improved thermal insulation of the central element and improvedsensitivity of the detector 100.

At the end of the method, the one or more bolometers produced are placedin a sealed enclosure.

According to a second embodiment, the carrier layer 32 can be used toform the thermometric element of the bolometer of the detector 100, andthe absorbing element of the detector can be formed from one or more MIM(Metal-Insulator-Metal) structures made on the membrane of thebolometer. FIG. 2 shows a detector 100 obtained by implementing a methodaccording to said second embodiment, wherein the reference numeral 56refers to a MIM structure which is formed from a lower metal portion 58,a dielectric portion 60 and an upper metal portion 62. A dielectricportion 64, comprising a dielectric material capable of withstanding thefinal HF etching, such as AlN or Al₂O₃, is positioned between the lowermetal portion 58 and the layer 32 in order to electrically insulate thethermometric element formed by the layer 32 relative to the lower metalportion 58. This dielectric portion 64 is, for example, made bydepositing a layer of the material of said portion 64 prior to thedepositing of the materials of the MIM structure 56, and is etched atthe same time as the materials forming the MIM structure 56. In thissecond embodiment, the MIM structure 56 is formed directly on the etchedlayer 32 forming the membrane of the bolometer. In this case, the partof the layer 32 forming the membrane assures the thermoelectric signaltransduction within the bolometer. The MIM structure 56 in particularprovides for the selective absorption of the wavelengths.

As in the first embodiment, the electrical connection between theconductive material present in the membrane of the bolometer and theread-out circuit of the detector is provided by means of the electrodes44 which directly cover the electrically conducting vias 38.

As a whole, the implementation of the aforementioned steps describedwith reference to FIG. 1A to 1J allow for the formation of a base forreceiving the different elements of the bolometers, i.e. the elementsperforming the optothermal and thermoelectric signal transductionswithin the bolometer. The different configurations of these elementsdescribed in document EP 2 743 659 A1 can apply when forming thedetector 100.

The production method described above can apply for the production of adetector 100 comprising a single bolometer or comprising a plurality ofbolometers.

The invention claimed is:
 1. A method for producing a bolometricdetector comprising at least: producing a stack of layers on anelectrical interconnect level of an electronic read-out circuit of thedetector, the stack comprising at least one sacrificial layer positionedbetween a carrier layer and a first etch stop layer, the first etch stoplayer being positioned between the sacrificial layer and said electricalinterconnect level, and the sacrificial layer comprising at least onemineral material capable of being selectively etched relative to thecarrier layer and the first etch stop layer; forming an opening in thestack of layers including etching through the carrier layer to expose anelectrically conducting portion of the electrical interconnect level ofsaid electrical interconnect level connected to the electronic read-outcircuit; producing at least one electrically conducting via in theopening passing through at least the stack of layers such that at leastone electrically conducting material of the via is in contact with atleast one electrically conducting portion of said electricalinterconnect level connected to the electronic read-out circuit;depositing at least one electrically conducting layer onto the carrierlayer and the via; etching the electrically conducting layer and thecarrier layer, forming a bolometer membrane electrically connected tothe via by at least one remaining portion of the electrically conductinglayer that covers at least one upper part of the via; and eliminatingthe sacrificial layer by chemical etching to which the first etch stoplayer and the carrier layer are resistant, and such that the membrane issuspended by means of the via.
 2. The method according to claim 1,wherein the stack of layers further comprises a second etch stop layersuch that the carrier layer is positioned between the second etch stoplayer and the sacrificial layer, the method further comprising, betweenthe production of the via and the deposition of the electricallyconducting layer, the implementation of the following steps of: removinga layer of electrically conducting material formed on the second etchstop layer during production of the via, then eliminating the secondetch stop layer, and wherein the electrically conducting layer isdeposited such that the remaining portion of the electrically conductinglayer also covers the sides of the upper part of the via uncovered bythe elimination of the second etch stop layer.
 3. The method accordingto claim 1, wherein the carrier layer comprises at least one dielectricmaterial or a material wherefor at least one electric parameter variesaccording to the temperature thereof.
 4. The method according to claim1, further comprising, before the elimination of the sacrificial layer,the production of at least one element for absorbing the infraredradiation intended to be detected by the detector, on the membrane. 5.The method according to claim 4, wherein the absorbing element comprisesat least one MIM structure.
 6. The method according to claim 1, whereinthe electrically conducting layer is etched such that a plurality ofremaining portions of said electrically conducting layer form bolometerelectrodes and resistive portions capable of absorbing infraredradiation intended to be detected by the detector.
 7. The methodaccording to claim 1, further comprising, between the etching of theelectrically conducting layer and the etching of the carrier layer, thedeposition of a first thermoelectric signal transduction layer on theremaining portions of the electrically conducting layer and on thecarrier layer.
 8. The method according to claim 7, wherein the firstthermoelectric signal transduction layer comprises at least one materialthat is resistant to the chemical etching process implemented toeliminate the sacrificial layer.
 9. The method according to claim 7,further comprising, after the deposition of the first thermoelectricsignal transduction layer, the deposition of a second thermoelectricsignal transduction layer on the first thermoelectric signaltransduction layer, the thickness of which is greater than that of thefirst thermoelectric signal transduction layer.
 10. The method accordingto claim 1, wherein: the carrier layer comprises amorphous silicon,and/or if a first thermoelectric signal transduction layer is deposited,the first thermoelectric signal transduction layer comprises amorphoussilicon, and/or if a second thermoelectric signal transduction layer isdeposited, the second thermoelectric signal transduction layer comprisesamorphous silicon.
 11. The method according to claim 1, comprisingforming the bolometer membrane directly connected to the via.
 12. Themethod according to claim 1, wherein producing the at least oneelectrically conductive via comprises depositing at least oneelectrically conductive material into the opening passing through atleast the stack of layers to reach the at least one electricallyconducting portion to form at least one electrically conducting viaelectrically connected to the electrical interconnect level.